Cellulose ethers represent an important class of commercially important water-soluble polymers (WSPs). Examples of such WSPs include carboxy-methylcellulose (CMC), hydroxyethylcellulose (HEC), methylcellulose (MC) and hydroxypropylcellulose (HPC). By incorporating additional functional groups into such cellulose ethers, a wide variety of mixed cellulose ether derivatives can be produced. Currently, cellulose ethers are manufactured by reacting cellulose with appropriate etherifying reagents. The cellulose material used to prepare cellulose ethers is referred to as “furnish”. At present both purified cotton linters and purified wood pulp are used to manufacture cellulose ethers.
The ability of a water-soluble cellulose ether to enhance the viscosity of water is primarily controlled by its molecular weight, chemical derivatization, and substitution uniformity of the polymer chain. For many industrial applications, high molecular weight cellulose ethers giving high solution viscosity are desired to lower their use level in a given application. Hence, technical alternatives have been sought to increase the viscosity of cellulose ether solutions. Approaches to increase solution viscosity of cellulose ethers include non-covalent inter-chain crosslinking by grafting hydrophobic groups onto the cellulose ether backbone as well as ionic crosslinking of the appropriate cellulose ether chains by various suitable ionic agents. However, cellulose ethers containing grafted hydrophobes are more expensive and may not provide high solution viscosity in the presence of various surface-active agents typically present in many water-borne formulations. To crosslink cellulose ethers with a crosslinking agent, however, has limitations as the concentration of crosslinking agent and the crosslinking conditions must be carefully controlled to prevent the formation of excessively crosslinked water-insoluble species. In addition, many crosslinkers are highly reactive and toxic. Since both permanent (i.e. covalent) and temporal (e.g. hydrophobic or ionic) crosslinking ultimately yield nonlinear polymers their rheological characteristics differ from those of their linear analogs. Thus for many industrial applications involving the use of cellulose ethers in aqueous media it is important to prepare high molecular weight cellulose ethers. In principle, this can be achieved by preserving the cellulose molecular weight during the manufacture of cellulose ethers, provided the starting cellulose has very high molecular weight. Unfortunately commercially available cellulose furnishes that are currently used to manufacture cellulose ethers have molecular weights lower than that of naturally occurring cellulose. Hence, they are unsuitable to make high molecular weight cellulose ethers.
Cellulose is a naturally occurring polymer and exists in a fibrous form in plants. Chemically, it is a homopolymer of anhydroglucose units connected through 1,4-beta-glycosidic linkages. Each anhydroglucose unit has three hydroxyl groups that are reactive to etherifying agents. In the natural form, it has the highest molecular weight.
The purest natural cellulose is the cotton lint or staple fiber which on a dry basis consists of about 95 wt % cellulose. However, due to its high cost cotton staple fiber is not used to manufacture cellulose derivatives. Currently, cellulose materials used to manufacture cellulose derivatives are isolated from trees or raw cotton linters. Cellulose fibers obtained by purification of wood are called wood pulps. Due to their low cost these furnishes are the most commonly employed sources of cellulose for the manufacture of cellulose derivatives.
Raw cotton linters have been considered an excellent source of high molecular weight cellulose for over 80 years. Raw cotton linters, commonly referred to as “linters”, are short fiber residues left on the cottonseed after the longer staple (“lint”) fibers are removed by ginning. Linters are shorter, thicker, and more colored fibers than staple fibers. They, also, adhere more strongly to the seed relative to staple fibers. Linters are removed from cottonseeds using a number of technologies including linter saws and abrasive grinding methods, both of which yield suitable materials. Depending on the number of passes used to remove the linters from the cottonseed, they are called “first-cut”, “second-cut” and “third-cut” raw cotton linters. If the linters are removed in one pass or first- and second-cut linters are manually blended in a weight ratio of approximately 1:4, the resulting material is called “mill runs”. Mill runs and first-cut raw cotton linters are used in medical and cosmetic applications as well as to make upholstery, mattresses, etc. while second cut cotton linters are typically used to manufacture purified cotton linters or chemical cotton. In general, first-cut cotton linters contain less non-cellulosic impurities than do second-cut cotton linters. The amount of hemicellulose, lignin or colored impurities and foreign matter in the various types of raw cotton linters increases in the following order: First-cut<second-cut<third-cut. Typically, the cellulose content of raw cotton linters is about 69-78 wt % as measured by the American Oil Chemists' Society (AOCS) “bB 3-47: Cellulose Yield Pressure-Cook Method”.
Another class of short length raw cotton linters collected from cottonseeds by beating the fiber-laden hulls of the cottonseed in a defibrillator is called “hull fiber”. To a lesser extent, acid is sometimes employed to remove linters. Linters resulting from this process are generally less desirable, unless one seeks to make a low molecular weight cellulose derivative, since the acid treatment can lead to molecular weight degradation.
In the past, the use of cotton linters in chemical processes was only after extensive mechanical and chemical cleaning to yield a high purity furnish. Purified cellulose obtained from raw cotton linters is called chemical cotton or purified cotton linters. Given the commercial significance of cotton, it is not surprising that many mechanical separation processes have been developed over the past century to separate lint and linters from other contaminants.
Regrettably, during the isolation and purification of cellulose from raw cotton linters or wood chips significant molecular weight loss of the cellulose occurs depending on the process conditions used to isolate the cellulose. In wood, due to the high concentration of other components, the molecular weight loss during purification is especially acute. In addition, due to oxidation caused by a bleaching process during the purification, undesirable functional groups, such as carboxyl or carbonyl groups are formed on the cellulose backbone and the polydispersity of the cellulose chains changes. Another drawback of purifying raw cotton linters to make chemical cotton or converting wood chips to wood pulp is that the crystallinity and morphology of the “virgin” cellulose fibers change leading to changes in chemical reactivity of the hydroxyl groups present in cellulose. Such an alteration of the cellulose microstructure could lead to changes in its reactivity or accessibility to a modifying agent and/or the formation of modified derivatives having different structures and different behavior in an end-use application. Notably, the processing associated with such purification greatly increases the cost of purified cellulose. Hence, from an economic standpoint, the manufacture of cellulose ethers from raw cotton linters is an attractive alternative.
To manufacture high quality cellulose ethers, it is critical to control the physical properties of the cellulose furnish, such as the level of impurities, surface area (fiber length), and crystallinity. Since cellulose is a semi-crystalline material one of the key issues in the manufacture of cellulose ethers is to make the cellulose hydroxyl groups equally accessible for reaction with the derivatizing agent. Since Mercer's original work on treating cellulose with caustic, a variety of methods have been developed over the past 150 years to render cellulose morphology more accessible to reactants. These approaches fall into three general categories: degradative treatments, mechanical treatments, and swelling treatments. In current commercial processes hydroxyl accessibility and reactivity are achieved by activating cellulose with an alkaline reagent, most typically sodium hydroxide.
Degradative treatments such as exposure of cellulose to a mixture of alkali and oxygen or mechanical means are generally undesirable, since they results in a loss of critical properties (e.g. viscosity) for many applications. Most commercial processes employ both mechanical and swelling treatments in an effort to enhance accessibility of all the cellulose hydroxyls present in the anhydroglucose units.
Although solvent-based syntheses of cellulose ethers in which the cellulose is fully dissolved in a nonreactive solvent are well known in the literature, such approaches are not practiced commercially because of the cost and environmental and recovery issues associated with the solvents. Commercial cellulose derivatization processes make use of heterogeneous reaction conditions by employing nonreactive organic diluents and/or high solids reactors.
In the slurry process, cellulose fibers are suspended (slurried) in a nonreactive organic solvent or a mixture of organic solvents, activated with a base solution, and etherified using the appropriate reagent. Slurry concentration is defined as the weight fraction of cellulose in the total reaction mixture. Typically, existing commercial slurry processes to manufacture cellulose ethers are run at about 4-9 wt % cellulose slurry concentrations using cut purified cellulose fibers. The “hairy” fiber morphology often present in purified cellulose furnishes coupled with their low bulk density precludes them from being handled and uniformly derivatized at greater than 9 wt % slurry concentrations.
The choice and operability of slurry concentration are dictated by the following:                a) Fiber length of the cellulose,        b) Bulk density of the cellulose, and        c) Ability to mechanically stir the cellulose slurry to bring about uniform distribution of reactants and efficient heat transfer.        
The water-solubility and performance characteristics of a given cellulose ether are dictated by the slurry characteristics used to make it. In general, high quality cellulose ethers are difficult to make at higher slurry concentrations due to non-uniform modification of the cellulose matrix. In general, slurry processes are preferred to high solids processes to manufacture certain types of cellulose ethers to achieve good stirrability, heat transfer, and quality products (no insolubles) having water-solubility with the least amount of etherification, as measured by the degree of substitution.
In slurry processes, the cellulose slurry concentration dictates the throughput of the cellulose derivatives made. The higher the slurry concentration the higher the throughput and, hence, the lower the manufacturing (mill) cost. Thus, from an economic point of view, the ability to make high molecular weight cellulose ethers at higher slurry concentrations than currently being practiced is desirable.
In the high solids process, the cellulose is activated with a base solution with little or no organic solvent to form a paste followed by etherification. In general, it is more difficult to obtain a uniform reaction of the etherifying agent in high solids processes due to the difficulties in achieving uniform mixing/distribution of reagents. In slurry processes, the free nonreactive liquid diluent provides a low viscosity medium to aid in the mixing of reactants. In both processes, reduction of fiber length improves mixing of reagents.
One of the drawbacks of the slurry process to manufacture cellulose ethers in an economical way is the inability to make cellulose ethers at greater than 9 wt % slurry concentration. In principle, by substantially reducing the fiber length of cellulose by cutting, it is possible to make cellulose ethers at slurry concentrations greater than 9 wt %. However, extensive cutting of cellulose required to shorten the fiber length is expensive and occasions molecular weight loss of the cellulose. In many applications, the molecular weight loss occurred during cutting of cellulose is undesirable as it calls for higher use level of the cellulose ether to achieve certain application properties. Another consideration for commercial processes is reactor loading: In order to maximize production output, it is desirable to produce as much product as possible for a given charge of the reactor. One of the means for addressing these issues involves cutting the cellulose fibers to increase bulk density. This approach permits increased reactor loading as well as improved mixing within the reactor.
A variety of approaches have been proposed to increase the bulk density of cellulose fibers by shortening fiber lengths. One approach for increasing the bulk density of cellulose fibers involves cutting or comminuting. U.S. Pat. No. 5,976,320 discloses a process of producing paper pulp by suspending in water a fibrous material such as cotton fiber and sending to a refiner, such as a conical or disk refiner in which the fiber material is beaten and thereby shortened and fibrilled. The shortened fiber material is then bleached and processed into a homogeneous paper pulp.
Another approach to increase the bulk density of cellulose and increase the solution rheology of cellulose ethers prepared from cellulose pulp is described in U.S. patent application No. 2002/0103368 A1. The invention involves a method of preparing cellulose floc comprising the steps of (a) obtaining mercerized and recovered cellulose pulp, and (b) treating the mercerized pulp to form the cellulose floc. The cellulose floc prepared by this method was found to have a higher bulk density than cellulose floc prepared from similar non-mercerized floc. The solution viscosity of carboxymethylcellulose (CMC) produced from mercerized and recovered cellulose pulp is significantly greater than that produced from non-mercerized cellulose pulp.
U.S. Pat. No. 2,663,907 discloses a process in which cellulose is embrittled at elevated temperature and converted to high density powder by passing it through two rotating rolls under high pressure. Unfortunately, this particular process results in degradation of the cellulose chains, with the resulting low molecular weight cellulose furnish being unsuitable as a feedstock for the manufacture many cellulose derivatives.
Jet mills have been utilized to reduce the particle size of cellulose and cellulose ethers, although in the case of linters the degree of polymerization noted in an example decreased by 26 wt %. However, this approach causes significant loss of molecular weight of cellulose when cotton linters are milled.
As part of a process for producing tissue paper, Paterson-Brown et al. in U.S. Pat. No. 6,174,412 B1 disclose a process for preparing cotton linter-based pulp in which the linters are initially mechanically cleaned, digested with caustic, bleached, and refined with a Hollander type beater over a period of several hours until the average fiber length is within the range of 0.3-3.0 mm.
Finely powdered or cut celluloses have been previously used in both batch and continuous processes to form activated cellulose that is then subsequently reacted with appropriate reagents to form a variety of cellulose derivatives.
Methylcellulose and its mixed hydroxyethyl- or hydroxypropyl-ethers prepared from raw cotton linters have been disclosed. German Patent Application No. 4,034,709 A1 describes the preparation of high molecular weight methylcellulose, ethylcellulose and hydroxyalkyl alkyl celluloses from raw cotton linters. U.S. Pat. No. 5,028,342 describes the use of a mixture of 20 to 80% by weight of carboxymethyl cellulose and 80 to 20% by weight of at least one polycarboxylic acid selected from a homopolymer of acrylic acid, a homopolymer of methacrylic acid, and/or copolymer of acrylic acid and methacrylic acid and/or salts thereof in drilling muds. It was mentioned that the low viscosity, crude (technical grade) carboxymethylcellulose (CMC) (DS about 0.9-1.3) was obtained from raw cotton linters and/or wood cellulose by the slurry process. However, the details of the preparation of the CMC and the slurry concentration used to prepare the CMC were not disclosed.
While some of the above publications describe the preparation of various types of cellulose ethers from raw cotton linters, none of them describes the preparation of cellulose ethers from high bulk density raw cotton linters. Such materials provide a unique composition that are especially well suited for the commercial manufacture of premium quality cellulose derivatives at greatly reduced cost using both slurry and high solids processes since they permit increased utilization of plant assets without additional investment.